The invention disclosed and claimed herein is generally directed to an arrangement for substantially reducing the transfer or transmission of mechanical vibrations between a magnetic resonance (MR) imaging system and the floor, walls and other structure of the building environment in which the MR system is sited. More particularly, the invention is directed to an arrangement of the above type for reducing transmission of vibrations in both directions, that is, from the MR system to surrounding structure, and also from surrounding structure to the MR system. The invention may include means for determining whether vibrations present at a site, if applied to the MR system, would adversely affect images produced thereby.
As is well known by those of skill in the art, MR imaging systems employ electrically excited coils to impose time varying magnetic fields on the static primary B0 field produced by the system main magnet. The imposed fields have associated currents which flow through conductors. Since these currents occur within a magnetic field, corresponding forces are applied to the conductors, which cause dynamic motions to be propagated throughout the MR system. Moreover, typical current waveforms contain repetitive pulses with fast transitions that produce vibrational energy within the audio frequency range. This causes the MR imaging system or scanner to radiate sound pressure waves, which may be very disturbing to both patients and system operators. In addition, MR systems now produce significantly higher levels of noise which is not related to the imaging or scanning process. The increased non-scanning noise levels result from the use of more powerful cryocoolers to cool the main magnet.
Both the scanning related and non-scanning related vibrational energy produced by an MR scanner may be transmitted through the base of the scanner into the floor or other horizontal surface which supports the scanner at the site of use, such as a hospital or other health care facility. The vibrations may be transferred from the supporting floor to adjacent building structure, and then be propagated therethrough to adjoining rooms, where it is radiated at levels which exceed allowable noise levels. Such structure-born acoustic noise is of increasing concern, as MR scanners become smaller and lighter and can thereby be installed and used in closer proximity to non-MR areas, such as patient rooms and staff offices. It is anticipated that regulatory limits on the allowable acoustic noise levels in such areas will become even more restrictive in the future.
Vibrations in the building structure adjacent to an MR scanner, which are transmitted into the base of the scanner through the supporting floor, are also of concern to the designers and users of MR imaging systems. Typical sources of such vibrations include fans and other air moving equipment, and motor/generator sets. Motion of system components resulting from these vibrations may induce eddy currents which disrupt the delicate frequency tuning involved in image generation/reconstruction. More particularly, the transmitted vibrations may cause relative motions between the various subassemblies of an MR system, such as the main magnet coils and thermal shields. Since these motions cause electrically conductive paths to move with respect to a magnetic field, they induce eddy currents, which in turn cause corresponding changes in the net magnetic field. Typical image degradation artifacts include phase ghosts, which are caused when the time varying magnetic fields induce unbalanced phase shifts in the precession of the RF excited molecules.
Efforts to control the flow or transfer of vibrational energy between an MR scanner and its support surface, in both directions, have encountered a number of complicating factors. Such vibrational energy tends to be divided between two different frequency ranges. Also, there is a large variation in structural characteristics of different MR sites. For example, the transmission of vibration tends to be much different for a scanner installed on a concrete slab at grade level than for a scanner mounted in a mobile van. Accordingly, data pertaining to the transfer of vibrational energy at one type of site would not be particularly relevant for a different type of site.
In the past, one approach to reducing adverse effects of vibrational energy flowing into an MR scanner was to design scanners so that they had a low sensitivity to the vibration spectrum which causes image degradation. Typically, such spectrum includes frequencies of 50 Hz and below. However, such low sensitivity requires very stiff attachment of all conductive parts of the MR scanner, and tends to have a number of undesirable consequences, such as increased cryogen consumption.
The invention is generally directed to apparatus for providing vibration isolation between an MR imaging system and an associated horizontal support surface, such as the floor in a hospital or other facility in which the MR imaging system is set up for use. The apparatus comprises a stiff platform of substantial mass which is provided with a bearing surface disposed to carry the entire weight of the MR imaging system. For example, the platform may have a mass which is approximately equal to the entire mass of the MR imaging system. The dimensions of the bearing surface are sufficiently large to accommodate the entire MR system xe2x80x9cfootprintxe2x80x9d, that is, the silhouette of the underside thereof. The apparatus further comprises a number of vibration isolation elements positioned to support the platform and the MR imaging system upon the horizontal support surface. Each of the isolation elements comprises an air-tight enclosure containing air under pressure, and is disposed to dampen vibrations and to thereby oppose the transmission of vibrations between the platform and the support surface. A pressure regulator is coupled to respective isolation elements to maintain specified air pressure levels therein, as required to support the platform in selected spaced-apart relationship above the horizontal support surface. Preferably, each of the isolation elements includes a side wall, such as a cylindrical member, which is formed of resilient material and is provided with an upper load bearing plate disposed to engage the platform. The load bearing plate of a given isolation element is positioned at a height above the support surface which is determined by the air pressure within the given isolation element. Thus, the platform may be maintained at a specified height above the support surface, and in a specified orientation such as a horizontal orientation, by operating the pressure regulator to maintain a specified air pressure level in each of the isolation elements.
In a useful embodiment of the invention, a shaker or other vibration generator is placed on the platform to apply mechanical vibrations of varying amplitudes and frequencies to the MR imaging system. A number of vibration sensors, such as accelerometers, are joined to the MR system to acquire data representing the applied vibrations, as well as the effects thereof on MR imaging. Because of the vibration isolation provided by the platform and the isolation elements, the acquired data will represent only the controlled vibrational energy produced by the vibration generator. Thereafter, when the MR system is set up at a hospital or other site of operation, the system is initially placed directly on the supporting floor. The vibration sensors are then employed to acquire a second set of data, representing vibrational energy at the site which is transmitted to the MR system through the floor. By comparing the two sets of acquired data, the MR system users will be able to readily determine whether the transmitted site vibrations will have a significant effect on images produced by the MR system. If the site vibrations do have such effect, the MR system may be placed on the stiff platform and isolation elements, as described above. Otherwise, it may remain on the floor of the site and be directly supported thereby, so that the platform and isolation elements will not be required.